Compare 2 images directly
In the compositor you can use the split viewer node.



From the manual:


  The SplitViewer node takes two images and displays one-half of each on each side (top socket on the right half, bottom socket input on the left). Use this node for making side-by-side comparisons of two renderings/images, perhaps from different render layers or from different scenes. When transitioning between scenes, you want to be sure the stop action is seamless; use this node to compare the end of one scene with the beginning of another to ensure they align.